CHAPTER XIII. 



SIMPLE AND COMPOUND EVOLUTION. 



§ 98. Where the only forces at work are those directly 

 tending to produce aggregation or diffusion, the whole his- 

 tory of an aggregate will comprise no more than the ap- 

 proaches of its components towards their common centre 

 and their recessions from their common centre. The pro- 

 cess of Evolution, including nothing beyond what was de- 

 scribed at the outset of the last chapter, will be simple. 



Again, in cases where the forces which cause movements 

 towards a common centre are greatly in excess of all other 

 forces, any changes additional to those constituting aggre- 

 gation will be comparatively insignificant — there will be 

 integration scarcely at all modified by further kinds of re- 

 distribution. 



Or if, because of the smallness of the mass to be inte- 

 grated, or because of the little motion the mass receives from 

 without in return for the motion it loses, the integration pro- 

 ceeds rapidly ; there will similarly be wrought but insignifi- 

 cant effects on the integrating mass by incident forces, even 

 though these are considerable. 



But when, conversely, the integration is but slow ; either 

 because the quantity of motion contained in the aggregate 

 is relatively great; or because, though the quantity of 

 motion which each part possesses is not relatively great, the 

 large size of the aggregate prevents easy dissipation of the 

 motion; or because, though motion is rapidly lost more 

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